Dynamic Fixation Device and Method of Use

ABSTRACT

A dynamic fixation device is provided that allows the vertebrae to which it is attached to move in flexion within the normal physiological limits of motion, while also providing structural support that limits the amount of translation motion beyond normal physiological limits. The present invention includes a flexible portion and two ends that are adapted for connection to pedicle screws. In at least one embodiment of the present invention, the normal axis of rotation of the vertebrae is substantially duplicated by the dynamic fixation device. The flexible portion of the dynamic fixation device can include a geometric shape and/or a hinge portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 10/435,330 filed on May 8, 2003, which claimed thebenefit of: U.S. Provisional Patent Application No. 60/379,167 filed May8, 2002 entitled “Dynamic Fusion System”; U.S. Provisional PatentApplication No. 60/390,181 filed Jun. 19, 2002 entitled “Dynamic FusionSystem”; and U.S. Provisional Patent Application No. 60/417,722 filedOct. 9, 2002 entitled “Dynamic Fusion System,” all of which areincorporated herein by reference in their entireties. Cross-referenceand incorporation thereof is also made to U.S. Nonprovisional patentapplication Ser. No. 10/406,895 filed on Apr. 4, 2003 entitled “DynamicFixation Device And Method of Use,” now U.S. Pat. No. 6,966,910.

FIELD OF THE INVENTION

This invention relates generally to securement devices and, moreparticularly, to a flexible rod or device along a portion thereof thatis capable of flexibly securing vertebrae together.

BACKGROUND OF THE INVENTION

The lumbar spine absorbs a remarkable amount of stress and motion duringnormal activity. For the majority of the population, the healingresponse of the body is able to stay ahead of the cumulative effects ofinjury, wear, and aging, and yet still maintain stability withreasonable function. In some cases, however, the trauma or stressexceeds the ability of the body to heal, leading to local breakdown andexcessive wear, and frequently also leads to local instability.Accordingly, degenerative change with age superimposed on baselineanatomy in the lumbar spine leads to problems including instability,pain and neurologic compromise in some patients. In some cases, thelocal anatomy may not provide the same protection to the motion segment,thereby aggravating this breakdown. Although rehabilitation,conditioning, the limitation of stress, and time to recover areeffective treatments for most patients, there is a significant failurerate with persistent pain, disability and potential neurologic deficit.

Referring now to FIGS. 1, and 2, two side views of a pair of adjacentvertebral bodies are shown. FIG. 1 illustrates two vertebra V₁ and V₂ ofthe spine in a neutral position. As shown in FIG. 2, when a person leansforwards, the spine undergoes flexion. The anterior portion of the spinecomprises a set of generally cylindrically shaped bones which arestacked one on top of the other. These portions of the vertebrae arereferred to as the vertebral bodies VB₁ and VB₂, and are each separatedfrom the other by the intervertebral discs D. The pedicles P₁ and P₂comprise bone bridges which couple the anterior vertebral body VB to theposterior portion of each vertebra. At each intervertebral joint or discD, flexion involves a combination of anterior sagittal rotation and asmall amplitude anterior translation.

The intervertebral joint is a complex structure comprising anintervertebral disc anteriorly, and paired zygapophyseal jointsposteriorly. The disc functions as an elastic support and connectionbetween the vertebra, and allows for flexion and extension of the spine,as well as limited rotation and translation. The zygapophyseal jointsand associated anatomy allow for significant flexion and extension whileproviding constraints in translation and rotation.

The primary bending motion in the lumbar spine is flexion and extensionin an anterior/posterior plane. This occurs in the range approximating10-15 degrees of flexion and extension. In a young or normal lumbarspine, this motion occurs about an axis in the mid to posterior portionof the disc. This is associated with a distraction or subluxation of thefacet joints or posterior elements of 10-15 mm. This occurs not about apure axis, but about a neutral zone, or a centroid of rotationassociated with the lumbar disc. The normal elasticity of the disc,joints and ligaments, and the degree of play or freedom associated withthese joints, as well as the nature of the loads applied to the spinecontribute to the size of this region of rotation. In some cases, therecurrent loads and motion on the disc and associated trauma to disc andmotion segment exceed the natural rate of healing or repair of the body.In this situation, there is breakdown in the motion segment associatedwith loss of the normal axis of rotation. As increasing subluxationoccurs with segmental motion, there is a dramatic shift in the axis ofrotation with displacement occurring within the disc space or frequentlyto some point outside of the disc. Therefore, in the situation of afailing motion segment, there is breakdown in the centroid of rotationwith associated translation of the vertebral segments. This translationis allowed by both breakdown occurring in the disc and instabilityassociated with both wear and degeneration of the zygapophyseal joints.The underlying anatomy of the motion segment and joints allows forsignificantly greater stress on the disc and contributes to degenerationboth in the disc and joints.

Traditionally, surgical treatment has been directed at treating neuralcompromise, or if the pain, instability, or risk of instability isconsidered sufficient, a segmental fusion has been considered. Morerecently, stabilization procedures have been tried over the past severalyears including artificial discs and ligaments and elastomericconstructs to protect the spine. Arthroplasty techniques to maximizefunction and reduce the dynamic effects on adjacent segments are a morerecent approach with less follow-up as to long-term results. A challengein designing such a system is constraining motion in a normalphysiologic range.

Spinal fusion surgery is a method of fusing at least two mobile segmentsof the spine to knit them together as one unit and eliminate motionbetween the segments. Current spinal fixation systems offer severaldrawbacks. Rigid fusion constructs do not allow relative movementbetween the vertebrae that are fused using a construct comprising apedicle screw, connector mechanism, and rigid rod. Furthermore, rigidimplants are known to create significant amounts of stress on thecomponents of the construct, including the pedicle screws and the rod,as well as the bone structure itself. These stresses may even cause therigid rod to break. In addition, the stresses transferred to the pediclescrews may cause the screws to loosen or even dislodge from thevertebrae, thereby causing additional bone damage.

Artificial discs may replace a failing disc and approximate a normalcentroid or axis of rotation; however, placement of such a device istechnically demanding and replaces the normal disc with a mechanicalreplacement with uncertain long-term results. The artificial disc willbe subject to wear without the healing potential of the body to healitself.

It is also desirable with some patients to have a spinal implant systemthat allows the vertebral column to settle naturally under the weight ofthe human body. Human bone heals more readily under some pressure. In arigid spinal implant system, the patient's spinal column may beunnaturally held apart by the structure of the implant. It is possiblethat this stretching of the vertebrae, in relation to one another,results in delayed or incomplete healing of the bone.

Posterior devices placed with pedicle fixation may provide somestabilization, however, the natural motion of such devices does notnecessarily act to mimic normal physiology. In a healthy lumbar spinethe axis of rotation or neutral area for motion is situated near theinferior posterior third of the lumbar disc. A desirable artificialsystem would closely approximate physiologic motion. However, to date,posterior systems have failed to address these concerns.

Several existing patents disclose fusion devices. For example, U.S. Pat.No. 5,415,661 discloses a device that includes a curvilinear rod suchthat the implant supposedly restores normal biomechanical function tothe vertebrae of the spine receiving the implant. However, the '661patent does not disclose a device having structure other than acurvilinear shape that has a radius of curvature of between 0 to 180degrees. In addition, the '661 patent does not disclose the concept ofproviding an anteriorly projected pivot point that models the naturalarticulation of the subject vertebrae by using a structure that providesa virtual rotation zone substantially identical to the rotation zoneprovided by the patient's vertebrae. In addition, as seen in FIG. 3 ofthe '661 patent, the device disclosed in the '661 patent utilizes a body4 having a central section 10 having an anteriorly oriented positionrelative to its ends 6a, 6b.

U.S. Pat. No. 6,293,949 also discloses a spinal stabilization deviceintended for use along the cervical vertebrae, and intended to beinstalled along the anterior side of the vertebrae.

U.S. Pat. No. 6,440,169 discloses a device that attaches to the spinousprocesses of two vertebrae and has a leaf spring that allows the deviceto compress and then recover spontaneously after the stress has ceased.However, the '169 patent does not address a construct that includes ananteriorly projected pivot point that allows the vertebrae to articulatewhen the spine undergoes flexion.

In view of the above, there is a long felt but unsolved need for amethod and system that avoids the above-mentioned deficiencies of theprior art and that provides an effective system that is relativelysimple to employ and requires minimal displacement or removal of bodilytissue.

SUMMARY OF THE INVENTION

The present invention provides a device that can be implanted and thatprovides for a specified amount of forward bending motion, therebyallowing anterior sagittal rotation between the vertebrae that receivethe implant. Reference is hereby made for the incorporation of theconventional descriptive terms of motion and other content presented inClinical Anatomy of the Lumbar Spine and Sacrum by Nikolai Bogduk, thirdedition, published by Churchill Livingstone, 1999. Although anteriorsagittal rotation or flexion between vertebrae is normal, significantanterior sagittal translation or sliding motion between vertebrae isnot. Thus, by allowing some amount of rotational motion while protectingagainst translation, the patient's condition or injury can be protected,thus promoting the healing process, while subsequently providing someability to rotate one vertebra relative to an adjacent vertebra, therebyallowing for improved spinal motion following surgery and recovery.Accordingly, as described herein, various implants, including a numberof rod configurations having flexible portions are presented thatprovide a device having the ability to elongate and bend. Thus, it is afirst aspect of the present invention to provide a device thatelongates, and a second aspect of the present invention to provide adevice that bends. More particularly, the present invention is a dynamicfixation device that includes a flexible rod portion, wherein theflexible rod portion can include a geometric shape and/or a hingeportion. These dynamic fixation devices are constructed of a material ofan appropriate size, geometry, and having mechanical properties suchthat they bend, thus allowing the vertebrae associated with the implantto rotate relative to one another, similar to the movement of a naturalspine.

A dynamic fixation device is a quasi-flexible, semi-rigid fixationconstruct that allows some measure of motion between the vertebraeattached to the dynamic fixation device. Dynamic fixation of the lumbarspine provides means of protecting lumbar structures and allows forhealing without proceeding to a lumbar arthrodesis. The constraints onsuch a system are in some ways different than for a rigid or near rigidconstruct, such as that used for fusion.

At the present time, pedicle fixation is an accepted method of fixing tothe spine. In the situation of a lumbar fusion, a relatively rigidconstruct is appropriate to stabilize the spine and allow healing of thebony structures. In the situation of providing protection to the lumbarstructures, a flexible system is appropriate to limit but not stop themotion of lumbar elements. The flexible elements in such a system needto accomplish several objectives. The primary objective is to allowphysiologic motion of the spine, while protecting against excessive ornon-physiologic movement. A secondary consideration is to protect thepedicle fixation from undue stress that could loosen the fixation at itsbony interface.

The normal instantaneous axis of rotation of the lumbar spine occurstypically near the lower posterior third of the disc. Conventionalpedicle fixation of the spine typically places the fixation rod or plateat the dorsal aspect of the apophyseal joint or posterior to the joint.Therefore, it is appropriate to consider a construct that effectivelyshifts this rotation point anteriorly toward the physiologic axis.

A group of geometries exist, which if applied to a posterior device,will constrain the subluxation of the segment and maintain the rotationin or close to the normal zone or axis of rotation. The indication foruse is to constrain the stresses and motion within a range which willallow the body's normal healing response to maintain adequate competencein the motion segment to avoid development of instability or neurologicdeficit and minimize pain or arthritis. The important features allow formaintenance of physiologic motion without the abnormal subluxation ortranslation that are associated with a degenerating disc and contributeto further degeneration. Thus, it is a separate aspect of the inventionto provide a construct that limits excessive subluxation or translation.

Although the motion is complex related to the range of stresses whichmay be applied, it is nonetheless possible to provide a device so thatwhile in compression, movement is axial or accompanied by slight dorsaltranslation, and that while in flexion allows both separation ofposterior elements and slight ventral translation allowing rotationabout the posterior portion of the disc.

Accordingly, it is an aspect of the present invention to provide adevice that allows for some limited motion, thereby decreasing thestresses placed on the various component parts of the implant, as wellas the affected vertebrae. It is a further aspect of the presentinvention to provide a device whose motion is designed to model thebending motion of the spine. Several separate embodiments of the presentinvention accomplish such tasks.

It is a separate aspect of the present invention to provide a constructthat geometrically accommodates the human spinal anatomy, whileproviding a structural member that provides an anteriorly projected zoneof rotation.

In a first embodiment, an implantable elastomeric material may be used,or a surgically implantable alloy can be used that includes a geometricshape having a plurality of arms (e.g., four arms) with an interior openregion between the arms. In one example of this embodiment, thegeometric shape is rectangular, such that the arms of the geometricshape are situated at 90 degree angles relative to each other. Upondeformation due to flexion of the spine, the geometric shape deforms,and the 90 degree angles between the arms change such that the geometricshape expands and becomes a parallelogram. In a separate aspect of theinvention, the convergence segments of the arms include partiallycircular corners. Alternatively, the partially circular corners may beof a different shape, such as partially triangular. In a separate aspectof this embodiment, the inside surface of the interior sidewalls of thearms of the geometric shape have an interior surface that is at an angleof 90 degrees relative to a planar surface of the geometric shape.Attached to the exterior of the geometric shape near two opposingcorners are two rod arms. The rod arms allow the device to be connectedto connectors, which interconnect the device to pedicle screws. In aseparate aspect of this embodiment, each rod arm may be situated atdifferent angles and locations along the geometric shape, therebyinfluencing the location of the projected pivot point in the plane ofthe geometric shape upon flexion of the spine.

In yet a separate embodiment, a dynamic fixation device utilizes atleast two adjacent geometric shapes that act in an accordion manner;however, this embodiment serves to project the effective pivot pointanterior relative to the device. Therefore, the projected pivot pointmimics the natural rotational axis of the vertebrae to which the deviceis attached. In a modification of this embodiment, more than twoadjacent geometric shapes are combined to form the flexible portion ofthe device. One aspect of this embodiment and its modification is thatsmaller geometric shapes may be used with the addition of more geometricshapes. Consequently, a smaller profile dynamic fixation device can beprovided, while at the same time having an effective pivot point that isprojected anteriorly a sufficient distance to mimic the naturalrotational axis of the vertebrae to which the device is attached.

In yet a separate embodiment, a dynamic fixation device is provided thatincludes a modified geometric shape that serves as the flexible portionof the device. The modified geometric shape incorporates an opening orvoid space that allows the device to elongate and deform to accommodateflexion of the spine.

In a yet a separate embodiment of the invention, the dynamic fusiondevice includes a geometric shape with an interior hollow region,preferably having sloped interior sidewalls. This feature allows thedevice to bend in a direction transverse to the plane of the geometricshape. The angle of the interior sidewalls can vary depending upon thedesired amount of projection of the pivot point, which acts as a virtualaxis of rotation for the device.

While the dynamic fixation devices described herein act to naturallycontrol the axis or region of rotation within the device, it is alsoadvantageous to consider the disc as part of the construct. If the discis assumed to be competent as regards axial loads as opposed totranslational loads, this competence can be used to control the discheight and concomitantly, the anterior portion of the implant andvertebral construct. Thus, in yet a separate embodiment, this allows aposterior construct having a rotatable anterior-posterior segment toeffectively control translation within a specific range of motion of thesegmental construct. Although there is a slight translation allowed,this is well within the natural region of rotation. This embodimentpreferably includes a hinged portion having pin. If anterior-posteriorsegment or hinged arm is considered to be an elastomeric segment, itsfunction depends on the translational forces being less than required tocause buckling of this segment. Controlling the shape of cross-sectionof this segment can allow forward bending of the spine while stillmaintaining competence in compression in the range of forces encounteredin the implanted situation.

For the above described devices, first and second rod arms are attachedto either end of the flexible construct, with the other end of the rodarms attached to connectors, which in turn are connected to pediclescrews that are inserted into vertebrae of the spine. During flexion andextension each vertebra exhibits an arcuate motion in relation to thevertebra below. The center of the arc lies below the moving vertebra.The dynamic fusion device provides a device for allowing movement of thevertebrae, with a forwardly or anteriorly projected pivot location thatmodels and substantially aligns with the actual pivot point of rotationfor the vertebrae to which the device is attached. Accordingly, thedynamic fusion device of the present invention provides a bendable rodfor fusion that mimics the movement of the vertebrae of the spine.

The dynamic portions of the various embodiments of the present inventionlengthen as they are elongated and shorten as they compressed. Thischaracteristic allows the devices to be implanted in the spine with apedicle screw system, and while the actual construct is positioned welldorsal in the spine, it allows the spine to function as though therewere a flexible construct in the anterior column of the spine.

In use, a problematic spinal disc is initially identified by aphysician. During surgery, an incision is made through the skin andmuscle overlying the implant location of the spine. Then a first pediclescrew is inserted into a first vertebra and a second pedicle screw isinserted into a second vertebra. The surgeon then attaches the dynamicfixation device to the pedicle screws using either an adjustableconnector or an end connector that is integrally formed as a part of thedynamic fixation device.

Various embodiments have been described in this summary of the inventionbut such embodiments are by no means to be deemed limiting to the“present invention” and the detailed description, the figures and theclaims should be referred to in there totality to appreciate the truescope and breath of the present invention. Moreover, while much of theabove discussion has focused on devices and particular configurations,various aspects of the present invention relate to surgical methods,methods of making such devices and methods of use which are also to beunderstood as being part of the present invention.

Additional advantages of the present invention will become readilyapparent from the following discussion, particularly when taken togetherwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side perspective view of two vertebra in a neutral position;

FIG. 2 is a side perspective view of the two vertebra shown in FIG. 1 ina condition of flexion;

FIG. 3 a is a side elevation view of a first embodiment of a dynamicfixation device used in conjunction with pedicle screws;

FIG. 3 b is a side perspective view of the device shown in FIG. 3 aattached to two vertebra in a neutral position;

FIG. 3 c is a side perspective view of the device shown in FIG. 3 aattached to two vertebra in a flexed position;

FIG. 4 a is a side elevation view of a separate embodiment of a dynamicfixation device used in conjunction with pedicle screws;

FIG. 4 b is a side perspective view of the device shown in FIG. 4 aattached to two vertebra in a neutral position;

FIG. 4 c is a side perspective view of the device shown in FIG. 4 aattached to two vertebra in a flexed position;

FIG. 5 a is a side elevation view of a modification of the dynamicfixation device shown in FIG. 4 a used in conjunction with pediclescrews;

FIG. 6 a is a front perspective view of a separate embodiment of adynamic fixation device;

FIG. 6 b is a front elevation view of the device shown in FIG. 6 a;

FIG. 6 c is a rear elevation view of the device shown in FIG. 6 a;

FIG. 6 d is a side elevation view of the device shown in FIG. 6 a;

FIG. 6 e is a side perspective view of the device shown in FIG. 6 aattached to two vertebra in a neutral position;

FIG. 6 f is a side perspective view of the device shown in FIG. 6 aattached to two vertebra in a flexed position;

FIG. 7 a is a side elevation view of a separate embodiment of a dynamicfixation device used in conjunction with pedicle screws;

FIG. 7 b is a side perspective view of the device shown in FIG. 7 aattached to two vertebra in a neutral position;

FIG. 7 c is a side perspective view of the device shown in FIG. 7 aattached to two vertebra in a flexed position;

FIG. 8 a is a side elevation view of a separate embodiment of a dynamicfixation device used in conjunction with pedicle screws;

FIG. 9 a is a side elevation view of a separate embodiment of a dynamicfixation device used in conjunction with pedicle screws;

FIG. 9 b is a side perspective view of the device shown in FIG. 9 aattached to two vertebra in a neutral position;

FIG. 9 c is a side perspective view of the device shown in FIG. 9 aattached to two vertebra in a flexed position;

FIG. 10 a is a side elevation view of a separate embodiment of a dynamicfixation device used in conjunction with pedicle screws;

FIG. 10 b is a side elevation view of a portion of the device shown inFIG. 10 a;

FIG. 10 c is a side perspective view of the device shown in FIG. 10 aattached to two vertebra in a neutral position;

FIG. 10 d is a side perspective view of the device shown in FIG. 10 aattached to two vertebra in a flexed position; and

FIG. 11 is a side elevation view of a separate embodiment of a dynamicfixation device used in conjunction with pedicle screws.

The above listed drawings are not necessarily to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While the present invention will be described more fully hereinafterwith reference to the accompanying drawings in which particularembodiments and methods of implantation are shown, it is to beunderstood at the outset that persons skilled in the art may modify theinvention herein described while achieving the functions and results ofthis invention. Accordingly, the descriptions which follow are to beunderstood as illustrative and exemplary of specific structures, aspectsand features within the broad scope of the present invention and not aslimiting of such broad scope.

As noted above, at each intervertebral joint or disc D, flexion involvesa combination of anterior sagittal rotation and a small amplitudeanterior translation. The various embodiments of the present inventionallow for controlled rotation while limiting translation within anacceptable, normal physiological range.

Referring now to FIG. 3 a, a side elevation view of a first embodimentof a dynamic fixation device 10 is illustrated. The dynamic fixationdevice 10 includes a geometric shape 12 connected to a first rod end 14and a second rod end 16. First rod end 14 and second rod end 16 arepreferably connected to connectors 18 a and 18 b that, in turn, areconnected to pedicle screws 20. Pedicle screws 20 are inserted into thepedicles of vertebrae when the device is attached to the vertebrae of apatient. Connectors 18 a and 18 b can be of the type that are integrallyformed as part of first rod end 14 and second rod end 16, respectively.Alternately, one or both of the connectors can be a separate type ofconnector that can be selectively positioned along the length of firstrod end 14 or second rod end 16, respectively, such that first rod end14 and second rod end 16 are adjustable (e.g., slidably) within theconnectors prior to tightening the connectors to fixedly interconnectthe device 10 to the pedicle screws 20.

Still referring to FIG. 3 a, dynamic fixation device 10 is shown in aneutral position. As noted, the dynamic fixation device 10 includes ageometric shape 12 between first rod end 14 and second rod end 16. Morespecifically, in one embodiment dynamic fixation device 10 includes asubstantially rectangular or substantially diamond-shaped geometricshape 12 that has four arms 22 a, 22 b, 22 c and 22 d. To the interiorof arms 22 a, 22 b, 22 c, and 22 d is hollow region or opening 24. Inlieu of an open space, opening 24 can be formed of and/or covered by aflexible or an elastic-type webbing material (not shown).

In a separate aspect dynamic fixation device 10, the centerline ofgeometric shape 12 is offset relative to the longitudinal axis ofdynamic fixation device 10. More particularly, as shown in FIG. 3 a,dynamic fixation device 10 has a longitudinal axis L-L that passesthrough the centerline of first rod end 14 and second rod end 16.However, the centerline CL-CL of geometric shape 12 is offsetposteriorly to the longitudinal axis L-L of dynamic fixation device 10.This offset provides a preference for the dynamic fixation device 10 tobend in flexion, but resist bending in extension.

It is an aspect of this embodiment that the arms 22 a, 22 b, 22 c, and22 d of geometric shape 12 are situated desired angles (e.g., atapproximately 90 degree angles) relative to each other when device 10 isin the neutral position. For example, arm 22 a is situated at an angleof about 90 degrees relative to arm 22 b and arm 22 d. Likewise, arm 22c is situated at an angle of about 90 degrees relative to arm 22 b andarm 22 d. Upon deformation of geometric shape 12 due to flexion of thespine, geometric shape 12 deforms and the angles between the arms willchange.

Still referring to FIG. 3 a, in yet a separate aspect of dynamicfixation device 10 the convergence segments 26 between the arms includesreduced dimensions. More particularly, the dimensions of arms 22 a and22 b are smaller in the vicinity where arm 22 a joins arm 22 b.Likewise, the dimension of arms 22 b and 22 c are also smaller in thevicinity where arm 22 b joins arm 22 c. This is also the case for theconvergence segments between arms 22 c and 22 d, and between arms 22 dand 22 a. The decreased dimensions of the arms 22 a, 22 b, 22 c and 22 dat the convergence segments 26 allow additional flexibility between thearms. As shown in FIG. 3 a, the convergence segments 26 includepartially circular corners between the arms. Alternatively, thepartially circular corners may be of a different shape, such aspartially triangular (not shown). Thus, dynamic fixation device 10preferably includes narrowing or thinning of the arms in the vicinity ofthe convergence segments 26. It is to be further noted that convergencesegments 26 serve as elastomeric hinges for geometric shape 12.

As shown in the example illustrated in FIGS. 3 b and 3 c, first rod end14 is shown to remain essentially immobile. Second rod end 16 movesbetween a neutral or first position 28, as shown in FIG. 3 b, and aflexed or second position 30, as shown in FIG. 3 c. In moving betweenfirst position 28 and second position 30 dynamic fixation device 10elongates and it also rotates about an effective pivot point 32. Thegeometric shape 12 provides an effective pivot point 32 that is forwardor anterior of the longitudinal axis L-L of first rod end 14 and secondrod end 16. During movement between first position 28 and secondposition 30, dynamic fixation device 10 experiences deformation, wherebyit bends and it elongates.

In use, a surgeon first makes an incision and then inserts pediclescrews 20. Subsequently, first rod end 14 and second rod end 16 ofdynamic fixation device 10 are preferably interconnected usingconnectors 18 a and 18 b to pedicle screws 20 that are inserted intovertebrae V₁ and V₂ of the spine. During flexion and extension, eachvertebra exhibits an arcuate motion in relation to the vertebra below.The center of the arc lies below the moving vertebra. Dynamic fixationdevice 10 provides a device for allowing movement of the upper vertebraV₁ to a flexed or second position 30, with a forwardly or anteriorlyprojected pivot location 32, as compared to the location of thelongitudinal axis L-L of the device 10 when it is in the neutralposition.

In a modification of the embodiment shown in FIG. 3 a, the geometricshape 12 can be subdivided into four smaller rectangles (not shown) asopposed to one large rectangle. This modification of using four smallerrectangles to form a geometric shape still acts as a larger rectangle interms of its effective pivot point. In yet an alternate modification ofthis embodiment, geometric shape 12 can take the form of a rhomboid (notshown). In this modification, an effective pivot point would beprojected forward (or anterior) some distance of the dynamic fixationdevice. Accordingly, depending upon its construction, the geometricshape 12 allows the pivot point to extend beyond the limits of thedevice. When the dynamic fixation device 10 is implanted posterior thespinal vertebrae, the device nonetheless allows for a rotation pointsubstantially anterior the device. Thus, depending upon the geometry ofthe dynamic fixation device, and more particularly, the geometry ofgeometric shape 12, the present invention allows an effective pivotpoint 32 to be created that substantially corresponds to the naturalpivot point of the patient's spine.

Referring now to FIG. 4 a, a side elevation view of a separateembodiment of a dynamic fixation device 34 is shown. The dynamicfixation device 34 of FIG. 4 a utilizes two adjacent but connectedsubstantially geometric shapes 36 a and 36 b. Substantially geometricshapes 36 a and 36 b act as two accordion shapes that expand andflexibly bend forward as dynamic fixation device 34 is elongated androtated during bending of the spine. Arrow A depicts the generaldirection of motion of second rod end 16 during rotation and elongationof the dynamic fixation device 34.

Still referring to FIG. 4 a, in one preferred embodiment, substantiallygeometric shapes 36 a and 36 b include a plurality of arms.Substantially geometric shape 36 a includes an anterior arm 38 a and aposterior arm 40 a. Similarly, substantially geometric shape 36 bincludes an anterior arm 38 b and a posterior arm 40 b. Preferably,anterior arm 38 a interconnects to posterior arm 40 b by crossing arm42. Similarly, anterior arm 38 b interconnects to posterior arm 40 a bycrossing arm 44. Although not required, crossing arm 42 can be hingedlyconnected to crossing arm 44 using a pin 46 positioned along crossingarm 42 and crossing arm 44. As with dynamic fixation device 10 describedabove, narrowing or thinning of the arms in the vicinity of theconvergence segments 26 is preferred. An opening 24 a exists betweencrossing arm 42, anterior arm 38 a and posterior arm 40 a ofsubstantially geometric shape 36 a, and another opening 24 b existsbetween crossing arm 44, anterior arm 38 b and posterior arm 40 b. Inlieu of an open space, openings 24 a and 24 b can be formed of aflexible or an elastic-type webbing material (not shown).

FIGS. 4 b and 4 c show dynamic fixation device 34 in its neutral andflexed positions, respectively. The effect of the substantiallygeometric shapes 36 a and 36 b is to produce an anteriorly projectedeffective pivot point 32 that substantially matches the rotational pointof the vertebrae to which it is attached. Thus, the device of FIGS. 4a-4 c substantially limits translational displacement of the vertebraeto which it is attached, while still allowing some amount of flexion. Ingeneral, the bending occurring with flexion is equal to the angle changebetween anterior arm 38 a and anterior arm 38 b as the constructelongates. Preferably, there is a rigid connection between first rod end14 and anterior arm 38 a, as well as a rigid connection between secondrod arm 16 and anterior arm 38 b.

In a separate aspect dynamic fixation device 34, the centerline ofsubstantially geometric shapes 36 a and 36 b is offset posteriorlyrelative to the longitudinal axis of dynamic fixation device 34. Moreparticularly, as shown in FIG. 4 a, dynamic fixation device 34 has alongitudinal axis L-L that passes through the centerline of first rodend 14 and second rod end 16. However, the centerline CL-CL ofsubstantially geometric shape 36 a and 36 b is offset posteriorly to thelongitudinal axis L-L of dynamic fixation device 34. This offsetprovides a natural fixation for the first rod end 14 to be acontinuation of anterior arm 38 a, and for second rod end 16 to be acontinuation of anterior arm 38 b.

Referring now to FIG. 5 a, in a modification of the embodiment shown inFIG. 4 a, more than two substantially geometric shapes may beincorporated into a dynamic fixation device 34′. More particularly, thedynamic fixation device 34 having substantially geometric shapes 36 aand 36 b may be modified to include a third, fourth, fifth, or anynumber of additional substantially geometric shapes. For example,substantially geometric shapes 36 a and 36 b of the device shown in FIG.4 a illustrate two substantially diamond shaped features, respectively.However, as shown in FIG. 5 a, a third substantially diamond shape 36 cmay be added to geometric shape 36 a and 36 b. Optional pins 46 may beused between the various substantially geometric shapes. Alternatively,four (not shown), five (not shown) or more geometric shapes may begrouped together to form a dynamic fixation device. Multiplesubstantially geometric shapes may differ in size and/or overall shapedconfiguration, which may be desirous depending upon the number used. Forexample, where three substantially geometric shapes 36 a, 36 b and 36 care used, as in dynamic fixation device 34′, the overall size of eachgeometric shape is preferably smaller than the two substantiallygeometric shapes 36 a and 36 b illustrated in dynamic fixation device34, as shown in FIG. 4 a. The, addition of added substantially geometricshapes projects the pivot pint 32 proportionally forward for the numberof substantially geometric shapes used.

Referring now to FIGS. 6 a-6 f, in yet a separate embodiment of theinvention, a dynamic fixation device 50 includes geometric shape 12 withan interior hollow region 24, wherein device 50 bends in a directiontransverse to the planar surface 52 of geometric shape 12. The interiorhollow region 24 preferably includes sloped interior surface 54. Thatis, the interior sidewalls 56 have an interior surface 54 that is at anangle θ. with the planar surface 52 of geometric shape 12. Angle θ ofinterior surface 54 can be one constant value, or it can vary within thedevice. By way of a non-limiting example, θ can be 60 degrees at the topof device 50, and vary to about 90 degrees at the bottom of device 50.

Referring now to FIGS. 6 a-6 c, interior hollow region 24 preferablyincludes four partially circular corners or convergence segments 26.Attached to two opposing partially circular corners or convergencesegments 26 are first rod end 14 and second rod end 16. Each rod end 14and 16 is situated at an angle of about 135 degrees from each adjacentside of the geometric shape 12. However, in an alternate aspect of thisembodiment, the rod ends 14 and 16 may be situated at different anglesrelative to the arms of the geometric shape 12. As with device 10,partially circular corners or convergence segments 26 may be of adifferent shape, such as partially triangular. Equivalently, amechanical hinge rather than an elastomeric hinge may be incorporated atconvergence segments 26.

As shown in FIG. 6 d, pedicle screws 20 are orientated perpendicular tothe planar surface 52 of geometric shape 12. Connectors 18 a and 18 bare used to attach the pedicle screws 20 to first and second rod ends 14and 16 of dynamic fixation device 50. The connectors 18 a, 18 b may beformed as an integral part of dynamic fixation device 50, or theconnectors 18 a, 18 b may be a separate device, as is known to thoseknowledgeable in the art. In use, the dynamic fixation device 50 expandsas it rotates and/or bends when attached to two vertebra that undergoflexion.

Referring now to FIGS. 7 a-7 c, yet a separate embodiment of a dynamicfixation device is shown. Dynamic fixation device 58 includes foursubstantially straight and rigid arm segments. These consist of lowerarm 60 a, first middle arm 60 b, second middle arm 60 c, and upper arm60 d. Lower arm 60 a and upper arm 60 d connect to connectors 18 a and18 b, respectively, which are then connected to pedicle screws 20. Usingpins 46, lower arm 60 a is hingedly connected to one end of middle arms60 b and 60 c. Upper arm 60 d is hingedly connected using pins 46 to theopposite end of middle arms 60 b and 60 c. Between the four hinge pointsis an opening 24 that is a quadrilateral shape. During flexion, upperarm 60 d moves upward and forward, thereby forcing middle arms 60 b and60 c to rotate downward. Thus, the hinged connection of middle arms 60 band 60 c to upper arm 60 d allows it to move forward, while theconnection of middle arms 60 b and 60 c to lower arm 60 a preventsexcessive translation or over-rotation. Dynamic fixation device 58allows for the upper vertebra to move up and forward, yet resistsexcessive translation of the vertebrae to which it is attached.

Referring now to FIG. 8 a, yet a separate embodiment of a dynamicfixation device is shown. The dynamic fixation device 62 shown in FIG. 8a is a dynamic fixation device that features an anterior-posteriorsegment 64. The dynamic fixation device 62 includes a first rod end 14having a rod arm 65 that extends at an angle α toward ananterior-posterior segment 64. Angle α is fixed in relation to pediclescrew 20 by the rigid connection between rod arm 65 and lower pediclescrew 20. Similarly, rod arm 73 is fixed by a rigid connection to theupper pedicle screw 20. Rod arm 65 of first rod end 14 is connected toanterior-posterior segment 64 at bend 66. More particularly, bend 66forming the connection between rod arm 65 and anterior-posterior segment64 can be a continuous structural piece such that rod arm 65 andanterior-posterior segment 64 are essentially a contiguous solid pieceincluding bend 66. Alternatively, bend 66 may be a hinged connectionwith a pin that interconnects rod arm 65 to anterior-posterior segment64. Anterior-posterior segment 64 is separated from rod arm 65 by angleβ.

Still referring to FIG. 8 a, at bend 66, anterior-posterior segment 64extends posteriorly to bend 68. Middle rod segment 70 extends from bend68 at the posterior end of anterior-posterior segment 64 to bend 72 thatforms the connection to rod arm 73 of second rod end 16. Bend 72 formsthe intersection and the connection between middle rod segment 70 androd arm 73. Bend 72 can be a continuous structural piece such thatmiddle rod segment 70 and rod arm 73 are essentially a contiguous solidpiece including bend 72, or bend 72 can be a connection thatinterconnects middle rod segment 70 and rod arm 73. The middle rodsegment 70 is separated from the anterior-posterior segment 64 by angleΦ.

First rod end 14 and second rod end 16 preferably are interconnected topedicle screws 20 using connectors 18 a and 18 b, respectively.Connectors 18 a and 18 b can be formed as an integral part of the end ofdynamic fixation device 62, or they can be separate devices, as is knownto those knowledgeable in the art.

Still referring to the example of the present embodiment shown in FIG. 8a, dynamic fixation device 62 also has a longitudinal axis L-L that isdefined by the center of connectors 18 a and 18 b. Rod arm 65 generallylies anterior of longitudinal axis L-L, and middle rod segment 70generally lies posterior of longitudinal axis L-L, withanterior-posterior segment 64 having portions both on the anterior andposterior sides of longitudinal axis L-L.

It is an aspect of the present embodiment that bend 68 preferably actsas a hinge and is able to move down if the vertebrae to which thedynamic fixation device 62 is attached is placed in compression. Inaddition, bend 68 can move up to accommodate flexion of the vertebrae.This motion of bend 68 and the anterior-posterior segment 64 closelyapproximates the normal arc of motion of human vertebra. When incompression, bend 68 moves down along a lower arc path 74. Lower arcpath 74 is caused when dynamic fixation device 62 is placed incompression and anterior-posterior segment 64 moves toward rod arm 65,thereby decreasing the angle β. In a typical human patient, angle β maydecrease up to 30 degrees as bend 68 passes along lower arc path 74. Toachieve this motion, bend 68 of dynamic fixation device 62 preferablyincludes a structure to allow it to act as a hinge. Accordingly, bend 68may include a pin 75. As illustrated in FIG. 8 a, pin 75 is shown in theneutral position. However, in the compressed position, pin 75′ is shownin its lower position. When the vertebrae undergo flexion, bend 68 movesup along an upper arc path 76. Upper arc path 76 is caused when dynamicfixation device 62 elongates and anterior-posterior segment 64 movesupward, thereby increasing the angle β. In a typical human implant,angle β may increase up to 30 degrees as bend 68 passes along upper arcpath 76. For at least some patients, the neutral position foranterior-posterior segment 64 will be slanted downward from horizontal,with bend 68 positioned lower than bend 66. Thus, angle β would have alesser amount of allowable compression over flexion extension. In theelongation condition, pin 75″ is shown in its upper position. Incompression, angle Φ will decrease, and when the dynamic fixation deviceelongates during flexion, angle Φ will increase.

The various embodiments of the present invention allows a slight amountof translational motion of the vertebrae, but the amount oftranslational motion allowed is within the physiological limits ofnormal motion of the human vertebrae. For example, for the embodimentshown in FIG. 8 a, as pin 75 moves forward along lower arc path 74 andupper arc path 76, the vertebrae will undergo a slight amount oftranslational movement, as is evidenced by the position of pin 75′ and75″, which are moved slightly anterior or forward from the neutralposition.

Referring now to FIGS. 9 a-9 c, yet a separate embodiment of a dynamicfixation device is shown. Dynamic fixation device 78 includes threesubstantially straight arm segments. These consist of lower arm 80 a,first middle arm 80 b, and upper arm 80 c. Lower arm 80 a and upper arm80 c connect to connectors 18 a and 18 b, respectively, which are thenconnected to pedicle screws 20. Using a pin 46, lower arm 80 a ishingedly connected to one end of middle arm 80 b. The opposite end ofmiddle arm 80 b is hingedly connected (e.g., by a pin 46) to upper arm80 c. During flexion, upper arm 80 c moves upward and forward, therebyforcing middle arm 80 b to rotate downward. Thus, the hinged connectionof middle arm 80 b to upper arm 80 c allow it to upward with forwardrotation, while the connection between middle arm 80 b and lower arm 80a prevents excessive translation or over-rotation. Similar to functionof the anterior-posterior segment 64 in device 62, middle arm 80 b inthe present embodiment acts as an anterior-posterior segment that allowsa range of motion in flexion, yet prevents the vertebrae fromexperiencing excessive translation. Thus, dynamic fixation device 78allows for the upper vertebra to move up and slightly forward, yetresists excessive translation of the vertebrae to which it is attached.

Referring now to FIG. 10 a, yet a separate embodiment of a dynamicfixation device is illustrated. Dynamic fixation device 82 includes afirst rod member 84 connected to a first rod end 14 and a second rodmember 86 connected to a second rod end 16, wherein the first rod end 14and the second rod end 16 are interconnected to pedicle screws 20 usingconnectors 18 a and 18 b, respectively. First rod member 84 and secondrod member 86 anteriorly and posteriorly confine a spring 88. Inaddition, rails 90 confine spring 88 on the lateral sides, and rails 90also serve to interconnect first rod member 84 to second rod member 86.The structure of dynamic fixation device 82 provides for an articulateddevice that can also elongate, thus accommodating the naturalphysiologic motion of two adjacent vertebra when undergoing flexion. Thestructure and function of these components will be described in detailbelow.

Still referring to FIG. 10 a, first rod member 84 preferably includes aconcave surface 92 along its posterior side, wherein the concave surface92 of first rod member 84 assists in providing anterior confinement ofspring 88. Second rod member 86 preferably includes a concave surface 94along its anterior side, wherein the concave surface 94 of second rodmember 86 assists in providing posterior confinement of spring 88.

As noted above, rails 90 (shown in dashed lines) interconnect the firstrod member 84 to second rod member 86. Preferably, rails 90 comprise aplate 96 with hinge pins 46 situated through both ends of the plate 96.Plate 96 is shown in FIG. 10 b. In one preferred embodiment, first rodmember 84 includes a first notch 98 for receiving a first hinge pin 46.Similarly, second rod member 86 includes a second notch 98 receiving asecond hinge pin 46. Plates 96 span the confinement zone 100 of spring88 and interconnect first rod member 84 and second rod member 86 whilelaterally containing spring 88 between rod members 84 and 86 andpreventing the spring 88 for moving outside of the confinement zone 100.In a separate aspect of the present embodiment, rails 90 may be formedusing a single piece. That is, the plate 96 and hinge pin 46construction may be machined or otherwise constructed of a single piece.

By way of example and not limitation, preferably spring 88 is acylindrical shaped spring having a proper spring constant for thedynamic fixation device 82. In addition, spring 88 may also take theform of a resilient material, such as a properly sized silicone insertshaped, for example, as a disc or a sphere. During flexion motion of thespine, second rod member 86 moves up and forward. During this movement,the spring 88 rolls between the first rod member 84 and the second rodmember 86. Since the spring 88 rolls, friction between first rod member84 and second rod member 86 is minimal. Thus, the ability of the springto roll can be modified by adjusting the shape of the spring and theshape and texture of the interior walls of the confinement zone 100.More particularly, the shape and surface texture of concave surfaces 92and 94 of the first and second rod members 84 and 86, respectively, canbe modified to adjust the magnitude and ease of motion in elongation ofthe second rod member 86 relative to the first rod member 84. Since thespring 88 is cable of being compressed, it deforms, thereby allowingbending. The amount of compression is controlled by the springcharacteristics, such as the spring material type, diameter and wallthickness, as well as the shape of the confinement zone 100 and thetexture of the concave surfaces 92 and 94. With regard to the shape ofthe confinement zone 100, the concave surfaces 92 and 94 serve as thecompression surfaces of the confinement zone 100 for spring 88. Theshape of the curves of the concave surfaces 92 and 94 can be altered tocontrol the degree of spring compression as the construct elongates. Forexample, referring to FIG. 10 a, the curvature of concave surfaces 92and 94 can be flattened, thereby influencing the reaction of the spring88 within the confinement zone 100 during flexion extension.

Referring now to FIGS. 10 c and 10 d, dynamic fixation device 82 isshown both in its neutral position and it the flexed position,respectively. For purposes of clarity, the rails 90 are dashed in FIGS.10 c and 10 d. As compared to the neutral position shown in FIG. 10 c,the elongated position of FIG. 10 d illustrates that spring 88 hasrolled up and is also slightly compressed. The characteristics of thespring 88 are chosen such that some desired amount of compression of thespring is allowed during flexion; however, the spring 88 is stiff enoughsuch that unwanted amounts of translation of the vertebrae are resisted.

Dynamic fixation device 82 is allowed to elongate because second rodmember 86 is hingedly attached to first rod member 84, thereby allowingvertical motion of second rod member 86 relative to first rod member 84.Thus, the structure of dynamic fixation device 82 provides for anarticulated device that can elongate, thus accommodating the naturalphysiologic motion of the spine.

Dynamic fixation device 82 has application to providing segmentallyapplied motion control of the spine because each motion segmentdesignated to receive an implant can have a dynamic fixation deviceimplant customized through its dimensions and spring constant, therebygiving the patient controlled motion within a desired normal physiologicrange.

In a typical use to span two vertebra, the total length of the dynamicfixation devices 10, 34, 34′, 50, 58, 62, 78, and 82 may beapproximately 15 to 35 mm. The geometric shape portions or hingestructures of the dynamic fixation devices, preferably occupy thecentral region of the implant that bridges two vertebra. That is, thegeometric shapes or hinge structures occupy only a portion of theimplant, thereby allowing first rod end 14 and second rod end 16 to besolid rod segments that can be interconnected to a pedicle screw using aconnector device. For those devices comprising a geometric shape orhinged structure, these structures will typically occupy approximately15 to 20 mm of the total length.

For a dynamic fixation device spanning one joint, it will expand up toapproximately 5 to 10 mm in length, and will rotate forward up tobetween 5 to 10 degrees to accommodate flexion of the spine. Obviously,different size dynamic fixation devices may be used to accommodate thespecific needs of each individual patient. More particularly, arelatively large dynamic fixation device may be needed for a large man,while a relatively small dynamic fixation device may be needed for asmaller patient, such as child or a petite woman. However, a limitednumber of sizes may provide adequate coverage for the majority of thepatient population. For any given device, a potential elongation of thedynamic fixation device consistent with the desired flexion of thevetebral motion segment and associated distraction of the plane of thefixation device is anticipated.

Referring now to FIG. 11, yet a separate embodiment of a dynamic device110 is shown. This embodiment includes a reverse C structure 112 thatserves to provide dynamic device 110 with a mechanism that may bedeflected, thereby allowing rotation and elongation of the device. Voidspaces 114 and 116 in combination with the reverse C structure 112provide structural aspects to dynamic device 110 that allow it todeflect, rotate, and elongate. This configuration creates an effectivepivot point 32 that is projected anterior to the device itself, therebyallowing it to project the location of pivot toward the naturalvertebra's rotation location.

Materials used to construct dynamic device 110 preferably comprisemetals or metal alloys, and more preferably stainless steel or titanium.Thinned portions of the dynamic device 110 are preferably formed oftitanium or stainless steel and have dimensions that allow resilientproperties.

In a typical use to span two vertebra, the total length the dynamicdevice 110 may be approximately 25 to 30 mm, with the reverse Cstructure 112 occupying approximately 15 to 20 mm of the total length.The dynamic device 110 may be used to flexibly fuse a plurality ofvertebra. The reverse C structure 112 may be located at specific pointswhere bending of the spine is desired, while a rigid rod may be used atother locations desired by the physician. Where used, rigid portions maybe curved, thereby influencing the implanted location of the reverse Cstructure 112, and thus the effective pivot point 32.

For a dynamic device 110 spanning one joint, it will expand up toapproximately 5 to 10 mm in length, and will rotate up to between 5 to10 degrees. Obviously, different size dynamic devices 110 may be used toaccommodate the specific needs of each individual patient. Moreparticularly, a relatively large dynamic device 110 may be needed for alarge man, while a relatively small dynamic device 110 may be needed fora smaller patient, such as a petite woman. However, a limited number ofsizes may provide adequate coverage for the majority of the patientpopulation. For any given device, a potential elongation of the dynamicdevice 110 of approximately 20% is anticipated.

The reverse C structure 112 of dynamic device 110 preferably occupiesthe central region of the rod. That is, the reverse C structure 112occupies only a portion of the rod, thereby allowing first end 14 andsecond end 16 to be solid rod segments that can be interconnected to apedicle screws 20 using connectors 18 a and 18 b.

The dynamic fixation devices can be used to flexibly secure a pluralityof vertebra. Alternatively, the dynamic fixation devices can be locatedat specific points where bending of the spine is desired, while a rigidrod may be used at other locations desired by the physician. Where used,rigid rod portions may be curved, thereby influencing the implantedlocation of the geometric shape hinged structures, and thus theeffective pivot point.

The structures of the present invention are made from one or morematerials that possesses the appropriate strength characteristicsnecessary to withstand loading from the human body when used in medicalapplications. In addition, the materials are compatible with the humanbody. Preferably, materials include ceramics, plastics, metals, orcarbon fiber composites. More preferably, the materials are made fromtitanium, a titanium alloy, or stainless steel.

The above described alternative configurations offer different bendingcharacteristics. The dimensions will vary depending upon the specificdesign necessary for a specific patient. More particularly, thedimensions of geometric shapes and hinged devices will likely be biggerfor a large heavy man, as opposed to that needed for a small petitewoman. Furthermore, the type of material used to construct the dynamicfixation devices described herein will also impact the requireddimensions of the devices. Dynamic fixation devices described herein maybe made of a variety of materials, preferably metals or materialsdemonstrating resilient characteristics, and more preferably, a titaniumalloy or surgical stainless steel. Since different materials havedifferent strength and resilient properties, the type of material usedwill, in part, dictate the dimensions of the rod portion required toachieve a certain function in a specific patient.

Devices disclosed herein can also be made of thermal memory materials ormaterials that possess different elastic properties at varyingtemperatures. In this aspect of the invention, the subject component(s)may be heated or cooled to a desired temperature, implanted, thensubsequently allowed to cool or warm to the temperature of the ambientconditions that will exist during the usage period for the subjectdevice, namely, normal body temperature.

It is to be understood that the present invention may have applicationto medical devices other than spinal implants. For example, the presentinvention can be used in external fixator systems.

Furthermore, it is understood that the present invention has applicationoutside the medical field. The dynamic fixation device of the presentinvention is not limited to medical implants. The device could be usedin seismic dampening applications. Alternatively, the present inventioncould be used to secure any two objects, such as in linking mechanisms,and has application to any type of mechanical device with a movingconnection. Other applications, by no means exhaustive, may includeconnecting any articulated device, such as an implement connection to atractor. It may also be used in heretofore static type connectionapplications, such as attaching an antenna to a base structure. One ofskill in various of the construction arts will appreciate how to makeand use the present invention in view of the guidance provided herein(with respect to a surgical application) and in view of the figures setforth herein.

While various embodiments of the present invention have been describedin detail, it is apparent that modifications and adaptations of thoseembodiments will occur to those skilled in the art. However, it is to beexpressly understood that such modifications and adaptations are withinthe spirit and scope of the present invention, as set forth in thefollowing

1.-20. (canceled)
 21. A vetebral implant assembly for posteriorattachment to a first vertebra and a second vertebra of a spine of apatient to allow controlled bending of the first vertebra relative tothe second vertebra, comprising: a first pedicle screw adapted forposterior attachment to the first vertebra, and a second pedicle screwadapted for posterior attachment to the second vertebra; a firstconnector connected to the first pedicle screw and a second connectorconnected to the second pedicle screw; and a dynamic fixation member,comprising: a first rod arm connected to the first pedicle screw by thefirst connector, and a second rod arm connected to the second pediclescrew by the second connector; and a deformable portion connected to thefirst and second rod arms, the deformable portion including an uppermember with a first void space and a lower member with a second voidspace, wherein the upper member and the lower member further comprise avariable thickness; wherein, when implanted posteriorly in the patient,the vetebral implant assembly allows controlled bending motion whilelimiting sagittal translation of the first vertebra relative to thesecond vertebra, and wherein the controlled bending motion occurs abouta virtual physiologic axis of rotation that is anterior of thedeformable portion.
 22. The vetebral implant assembly of claim 21,wherein the void spaces each include at least one rounded inner corner.23. The vetebral implant assembly of claim 21, wherein during thecontrolled bending motion the deformable portion expands approximately 5to 10 mm in length and rotates between approximately 5 to 10 degrees.24. The vetebral implant assembly of claim 21, wherein the deformableportion includes a connecting member that interconnects the upper memberto the lower member, and wherein the upper member and the lower membercomprise thinned portions adjacent the connecting member.
 25. Thevetebral implant assembly of claim 24, wherein each of the upper andlower members comprise a top substantially anterior-posterior alignedarm and a bottom substantially anterior-posterior aligned arm borderingeach void space, and wherein the top and the bottom substantiallyanterior-posterior aligned arms each comprise thinned portions.
 26. Thevetebral implant assembly of claim 24, wherein the upper member, thelower member and the connecting member substantially comprise a reverseC shape.
 27. A vetebral implant assembly for posterior attachment to afirst vertebra and a second vertebra of a spine of a patient to allowcontrolled bending of the first vertebra relative to the secondvertebra, comprising: a first pedicle screw adapted for posteriorattachment to the first vertebra, and a second pedicle screw adapted forposterior attachment to the second vertebra; a first rod arm connectedto the first pedicle screw by a first connector, and a second rod armconnected to the second pedicle screw by a second connector; and adeformable portion connected to the first and second rod arms, thedeformable portion including an upper member and a lower member with aconnecting member that interconnects the upper member to the lowermember, wherein the upper member and the lower member further comprise avariable thickness; wherein, when implanted posteriorly in the patient,the vetebral implant assembly allows controlled bending motion whilelimiting sagittal translation of the first vertebra relative to thesecond vertebra, and wherein the controlled bending motion occurs abouta virtual physiologic axis of rotation that is anterior of thedeformable portion.
 28. The vetebral implant assembly of claim 27,wherein the upper member includes a first void space and the lowermember includes a second void space.
 29. The vetebral implant assemblyof claim 28, wherein the void spaces each include at least one roundedinner corner.
 30. The vetebral implant assembly of claim 28, whereineach of the upper and lower members comprise a top substantiallyanterior-posterior aligned arm and a bottom substantiallyanterior-posterior aligned arm bordering each void space, and whereinthe top and the bottom substantially anterior-posterior aligned armseach comprise thinned portions.
 31. The vetebral implant assembly ofclaim 28, wherein during the controlled bending motion the deformableportion expands approximately 5 to 10 mm in length and rotates betweenapproximately 5 to 10 degrees.
 32. The vetebral implant assembly ofclaim 27, wherein the upper member, the lower member and the connectingmember substantially comprise a reverse C shape.
 33. A deformable memberfor implantation in a patient by posterior attachment to a firstvertebra of a spine of the patient using a first pedicle screw and afirst connector, and to a second vertebra using a second pedicle screwand a second connector, to allow controlled bending of the firstvertebra relative to the second vertebra, comprising: a first rod armconnected to the first pedicle screw by the first connector, and asecond rod arm connected to the second pedicle screw by the secondconnector; and a deformable portion connected to the first and secondrod arms, the deformable portion including an upper member with a firstvoid space and a lower member with a second void space, wherein theupper member and the lower member further comprise a variable thickness;wherein, when implanted posteriorly in the patient, the deformablemember allows controlled bending motion while limiting sagittaltranslation of the first vertebra relative to the second vertebra, andwherein the controlled bending motion occurs about a virtual physiologicaxis of rotation that is anterior of the deformable portion.
 34. Thedeformable member of claim 33, wherein the void spaces each include atleast one rounded inner corner.
 35. The deformable member of claim 33,wherein during the controlled bending motion the deformable portionexpands approximately 5 to 10 mm in length and rotates betweenapproximately 5 to 10 degrees.
 36. The deformable member of claim 33,wherein the deformable portion includes a connecting member thatinterconnects the upper member to the lower member, and wherein theupper member and the lower member comprise thinned portions adjacent theconnecting member.
 37. The deformable member of claim 36, wherein eachof the upper and lower members comprise a top substantiallyanterior-posterior aligned arm and a bottom substantiallyanterior-posterior aligned arm bordering each void space, and whereinthe top and the bottom substantially anterior-posterior aligned armseach comprise thinned portions.
 38. The deformable member of claim 36,wherein the upper member, the lower member and the connecting membersubstantially comprise a reverse C shape.